Many surfaces are typically wetted by liquids. The degree of wetting is a function of the interplay between the forces of cohesion in the liquid and the forces of adhesion between the liquid and surface.
In many instances, wetting of surfaces is undesirable. For example, wetting of surfaces with water results in retention of water droplets on the surfaces. Upon evaporation of the water, solids dissolved or suspended in the water remain as unsightly residues on the surface. Wetting of surfaces with water also can act as a trigger for corrosion or for infestation of the surface with microorganisms, such as bacteria. Wetting surfaces can also lead to excess ice build up on surfaces in colder climates. In the context of packaging and storage vessels, wetting of interior surfaces results in retention of liquid within the packaging or storage vessel, leading to loss of liquid on transfer, or hold up in injection molding or related applications.
It is known that the wettability by hydrophilic liquids may be reduced by use of hydrophobic coatings on surfaces. Suitable coating materials include polysiloxanes and perfluorinated polymers, such as polytetrafluoroethylene (Teflon™). The coating reduces the forces of adhesion between liquid and the wetted surface.
In addition, it is known that the microscopic and nanoscopic architecture of the surface affects the adhesion of liquid droplets to surfaces. The leaf surfaces of lotus leaves and other plants have long been known to be superhydrophobic. These leaf surfaces are covered with microbumps (called papillae), which, in turn, are covered with hydrophobic wax with nanoscale roughness. These surfaces exhibit so-called “reentrant” surface topography that traps air underneath droplets, which serves to make macroscopically large contact angles by disrupting the droplet contact line (see, e.g., Öner et al., Langmuir, 16(20): 7777-7782 (2000); Tuteja et al., MRS Bulletin 33: 752 (2008)). Mimicry of the leaf surface structure has led to development of synthetic hydrophobic surface coatings having micro- and nanoscale surface structures formed thereon with a height and spacing on the order of 1-150 microns, where structural spacings in the range of 1-50 microns have shown the most consistent synthetic superhydrophobic effects (Öner, 2000). Known methods for making superhydrophobic materials include forming flat surface arrays of vertically aligned polytetrafluoroethylene coated carbon nanotubes, forming periodic arrays of micropillars on a flat surface using microelectronics based photolithography and hydrophobically modifying their surfaces, depositing self aligned polymer nanospheres onto surfaces, soft lithographic stamping or embossing of such structures into hydrophobic polymers, and using porous or roughened fluorinated polymers as superhydrophobic coating materials.
However, many of these superhydrophobic materials are costly to prepare and are insufficiently robust for use in real world applications. Thus, there remains a need in the art for improved superhydrophobic coatings.